One of the things I miss in Mail is the ability to set a reminder to respond to a mail message at a given time. So I re-wrote another script I'd found to do just this using iCal.
Open Script Editor, and then copy and paste the script into the editor. Save it as a script, anywhere you like (though I saved it in Mail's Script menu, for fast access).
To use it select a SINGLE mail, run the script and follow the dialogues. This script was done on the fly; I am sure it can be improved and trimmed.
[robg adds: This worked as expected. Note that the date format defaults to day-month-year. If you work with the US-style month-day-year format, you'll just need to change one line in the script. Here's how it looks after modifying it:
display dialog "Enter Date of the event for " & theFrom & ": i.e " & ¬
theMonth & "/" & theday & "/" & theyear default answer ¬
theMonth & "/" & theday & "/" & theyear
With this change, you'll both see and enter US-style dates.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20050616172843429